A method of treating an article with a water/oil repellent composition which comprises an emulsion having, dispersed in a medium, a copolymer comprising polymerized units derived from a monomer having a polyfluoroalkyl group (hereinafter referred to as a Rf group, in the present invention) is known as a method for imparting water/and oil repellency to a surface of the article (e.g. a fiber product). Such a water/oil repellent composition is required not to lose the water/oil repellency substantially even when repeatedly subjected to washing (durability against washing).
As a water/oil repellent composition with excellent water/oil repellency, the following water/oil repellent compositions have, for example, been proposed:
(1) a water/oil repellent composition containing a copolymer as an indispensable component, which consists essentially of polymerized units derived from the following monomer (a) and the following monomer (b) (Patent Document 1):
monomer (a): a monomer having a C1-6 perfluoroalkyl group (hereinafter referred to as a RF group, in the present invention), etc.;
monomer (b): a (meth)acrylate having an alkyl group having at least 15 carbon atoms, etc.;
(2) a water/oil repellent composition containing a copolymer, as an indispensable component, which contains polymerized units derived from the following monomer (a) and the following monomer (b) in an amount of at least 30 mass % and less than 80 mass % and further contains polymerized units derived from the following monomer (c) (Patent Document 2):
monomer (a): a monomer having a C1-6RF group, etc.;
monomer (b): a monomer having no Rf group and having a cross-linkable functional group;
monomer (c): at least one monomer having no Rf group (excluding monomer (b)), which contains the following monomer (c1) or (c2) in an amount of at least 50 mass % based on the total amount of monomer (c):
monomer (c1): a (meth)acrylate having a C16-40 alkyl group, etc.; and
monomer (c2): a monomer, of which the homopolymer has a glass transition point of from −50° C. to 40° C. (excluding monomer (c1)).
However, fiber products etc. treated with the water/oil repellent composition (1) or (2) have a problem such that their flexibility is insufficient and their texture is likely to be deteriorated. Additionally, when the water/oil repellent composition (1) or (2) is vigorously stirred, or foreign substances (such as dyestuffs) are added thereto, the emulsion of the composition tends to be unstable. Therefore, a water/oil repellent composition having mechanical stability and stability to foreign substances is desired.    Patent Document 1: WO02/083809    Patent Document 2: WO2004/035708